This invention relates to the control of variation in pitch of the blades of a fan, especially the fan of a double-flow reactor.
A number of different systems have already been employed for this purpose. A first system comprises toothed racks disposed in an axial direction and in meshing engagement with pinions rigidly fixed to the fan blades; by displacing the toothed racks in sliding motion either in one direction or in the other, the pitch of the blades is either increased or decreased. A system of this type is attended by a number of disadvantages. In the first place it is cumbersome and in particular cannot be housed between the blades when these latter are present in large number and consequently placed very close together. In the second place the system is subject to a certain play in the gear-teeth and gives rise to components of tangential forces.
In another known system, each blade is rigidly fixed to a bevel-pinion disposed simultaneously in meshing engagement with two toothed rings which are coaxial with the fan assembly as a whole and driven in opposite directions in order to carry out the variation of pitch. In a system of this type, the play in the gear-teeth can be reduced to zero since these latter can be subjected to prestress at the time of assembly. This system is also less cumbersome than the toothed-rack systems. However, the ring-gears are driven in rotation by means of a motor, usually of the hydraulic type, by means of gear-trains in which the teeth fail frequently due to fatigue since they are subjected to a very high unitary pressure by reason of the high value of the couples to be transmitted, especially when the fan has a large number of blades.